The present invention relates to a process for the production of molded bodies from silicon-infiltrated, reaction-bonded silicon carbide (SiSiC) for applications in machine building, in particular in the building of sliding rings and heat exchangers. The porous molded bodies comprise a mixture of silicon carbide and carbon and are heated in a vacuum in the presence of silicon to temperatures in excess of 1410.degree. C., above the melting temperature elemental silicon, with a holding time of from about 0.5 to 6 hours.
German Offenlegungsschrift No. 29 10 628 discloses the infiltration of porous molded bodies of a silicon-carbon mixture, with the use of silicon-containing feeders. These feeders are prepared from a mixture of carbon and silicon, wherein silicon forms, upon heating with the carbon, a skeleton of newly formed silicon carbide, and the molten silicon, which is supplied in excess, is introduced into the molded body to be infiltrated. The SiC matrix of the feeders, which as a result is porous, is readily removed after firing. It is desirable in the process to place the silicon carbide-carbon pressed part at the bottom and the silicon-carbon pressed part on top as the feeder, since this type of arrangement favors the flow of the molten silicon into the silicon carbide-carbon body, together with its penetration as a result of the capillary effect and of gravity. The disadvantage of this method resides in the fact that a mass of silicon and carbon must be specially prepared and pressed into a feeder, which cannot be reused. A variant of the preparation of the feeder employs the mixture of silicon and carbon for the siliconizing process in a loose power form. The amount desired is poured into and around the silicon carbide-carbon pressed shape. In both of these methods, fine grained silicon and carbon are mixed with each other. It has been found detrimental that impurities are introduced with the very fine silicon, having an average grain size of 75 .mu.m, which later may lead to defects, especially cracks in the finished, siliconized molded body.
In order to improve the impact strength of SiSiC molded bodies, 10 to 55% of boron nitride is added according to German Offenlegungsschrift No. 27 07 299 to the molded body formed of a mixture of silicon carbide and carbon. The boron nitride essentially does not react with the molten silicon. Siliconizing is effected by placing the molded body into a mold together with silicon which later is melted. The infiltration of the molten silicon is effected at temperatures in excess of 1410.degree. C. by means of wicks which are comprised of carbon fibers. Boron nitride may be used, for example, as the mold release agent. A disadvantage of the process is the very expensive preparation of the graphite mold and of the wick material, which renders it uneconomical for the mass production of molded bodies.
A further difficulty in siliconizing is encountered in German Offenlegungsschrift No. 26 44 503, because the commercially available silicon carbide powder always contains some silicon dioxide. This silicon dioxide is not wetted by the molten silicon metal so that siliconizing in the case of objects with a material of this type is always difficult. In this process the porous object is heated in the absence of oxygen to the siliconizing temperature above the melting temperature of silicon. The object is then maintained in a gaseous environment containing essentially nitrogen with from 0 to 10%, preferably 3 to 7% hydrogen contained therein. In this binary gas treatment nitrogen and hydrogen react with all of the oxides of silicon present in silicon carbide and convert them to silicon nitride. The nitrogen also reacts with clean silicon surfaces to form silicon nitride. The dual gas treatment thus effects a purification of the object by removing the silicon oxides, and also insures the availability of an adequate pore volume within the molded body, sine the carbon in the silicon carbide is converted in a manner so that the porous structure of the object is not prematurely sealed. This nitrogen containing environment is removed when the siliconizing temperature is attained so that silicon metal flows into the molded body over the porous skeleton and rapidly penetrates the available carbon and the graphite present in the object. If a nitrogen environment is present in the firing chamber, the nitrogen reacts with the liquid silicon metal to form a silicon nitride skin over it, which prevents any penetration of the porous molded body by the silicon.
Basically, all of the processes known from the literature for preparing infiltrated silicon carbide have the disadvantage that, upon solidification, silicon exudes from the molded body and wets its surface over large areas, which frequently leads to adhesion with the auxiliary firing agents. As a consequence, upon the separation of the molded bodies from the firing support the part is damaged. Furthermore, the often substantial mechanical or chemical cleaning effort required for the surfaces covered with silicon is highly uneconomical.